MX2007006440A

MX2007006440A - Method of controlling a wind turbine connected to an electric utility grid.

Info

Publication number: MX2007006440A
Application number: MX2007006440A
Authority: MX
Inventors: John Nielsen; Claus Esbensen
Original assignee: Vestas Wind Sys As
Priority date: 2004-12-28
Filing date: 2004-12-28
Publication date: 2008-03-11
Also published as: US20090079193A1; JP2008526179A; CN101091305A; EP1831987A1; ES2637890T3; CA2591598C; WO2006069569A1; US7859125B2; AU2004326154B2; EP1831987B2; BRPI0419255A; EP1831987B1; CA2591598A1; ES2637890T5; AU2004326154A1

Abstract

The invention relates to a method of controlling a wind turbine connected to an electric utility grid during malfunction in said electric utility grid (9). The method comprises the steps of detecting a malfunction in said electric utility grid and operating at least two control units of said power converter (12) in relation to at least one power converter limit value. The invention also relates to a control system for a wind turbine connected to a utility grid and a wind turbine.

Description

METHOD TO CONTROL AN EOLIC TURBINE CONNECTED TO AN ELECTRICAL DISTRIBUTION NETWORK FIELD OF THE. INVENTION The invention relates to a method for controlling a wind turbine connected to an electrical distribution network during some failure of the electrical distribution network, it is also related to a control system in accordance with the preamble of claim 10 and with a wind turbine according to the preamble of claim 15.

BACKGROUND OF THE INVENTION In general, wind turbines are connected to electric distribution networks in order to generate and supply electric power to consumers who are far from wind turbines. The electric power is sent to the consumers, by the transmission or distribution lines of the network. Wind turbines and other means of generating electrical energy connected to a distribution network are protected, in general, by disconnect switches, against faults in the distribution network.

The switches disconnect the wind turbines from the distribution network when the fault is detected. The fault can 52-442 defined as variations that occur in the distribution network and that are greater than a certain specific limit, for example, voltage drops greater than +/- 5% with respect to the nominal value of the distribution network voltage. Failures in the distribution network can include, in certain cases, several important voltage drops in a short period, for example, low voltage or weakening of illumination by low voltage that is among the most commonly recorded alterations in electrical energy. in the distribution networks. A problem that arises when the wind turbines are disconnected from the distribution network is the fact that the voltage variation can increase in magnitude or duration because the production of the electric power of the generators of the wind turbines is lost. In addition, a certain interval of time is required before the disconnected wind turbines can be reconnected to the distribution network. The disconnection of wind turbines affects the production of electricity from wind turbines and, thus, their profitability. In the prior art, different solutions have been suggested for a wind turbine to support short faults in the distribution network. However, it is possible that a modern variable speed wind turbine will be damaged when the distribution network voltage is suddenly interrupted and is not disconnected from the grid. It can suffer damage due to a rapid increase in voltage on the rotor side of the wind turbine generator or frequency converter. Additional damage can be caused to the wind turbine at the time when the mains voltage is recovered, due to the flow of large currents and, especially, in the frequency converter. The prior art, described in the German patent application no. DE-A 102 06 828, suggests the use of a resistor and a power transistor in the DC link between a rectifier and an inverter circuit and connected in parallel with the capacitor of the DC link. The resistor can be connected and disconnected to discharge the capacitor and thus eliminate a short voltage spike. One of the objects of the invention is to establish a technique for controlling a wind turbine when there are serious faults in an electrical distribution network, without presenting the aforementioned disadvantage. A special object of the invention is to generate a technique that is flexible and that, in this way, can protect the wind turbine during the failure of the distribution network, as well as immediately after the failure is eliminated, regardless of the nature this.

SUMMARY OF THE INVENTION The invention relates to a method for controlling a wind turbine connected to an electrical distribution network during a fault in the distribution network, the method comprises the steps of putting at least two control units of the power converter into operation with respect to at least one limit value of the power converter. Through this, a method is established that does not include the above-mentioned disadvantages. An advantage of the method is that it allows a more flexible control of the means of protection during network failures, in which, to face the failure of the network and its exact consequences, it can be chosen from a large number of different techniques. In particular, it is possible to reduce the dV / dt value and thus avoid any voltage or current peaks that could damage, for example, the power converter switches. In one aspect of the invention, the at least two control units are operated with respect to a minimum or maximum limit voltage value of the DC link in the power converter to maintain the value of the DC link voltage between the minimum and maximum limit voltage values. By means of the present, it is possible to remove or add control units with respect to a voltage, a temperature value or additional work values that represent the converter to face and suppress the consequences of the failure. In an aspect of the invention, the control units include the generator and the circuits, which are on the side of the network, of the power converter, which enter into operation to disconnect from the electric generator and from the electric distribution network to the power converter at the moment when the minimum or maximum limit value of the DC link is reached. By means of the present, it is possible to protect the power converter if the failure of the network is very serious to support it without disconnecting it from the distribution network. In addition, it is possible to keep some minimum values, such as the voltage values of the DC link and of the frequency converter, which will be converted into the initial working values of the power converter at the moment in which the distribution network recovers its normal functioning . In one aspect of the invention, the control units further contain one or more resistor blocks that connect at least one resistor between the DC link distribution busbars of the power converter. By means of the present, it is possible to direct the capacitor energy of the DC link to a ground plane through the resistor and thereby reduce the voltage of the DC link. In one aspect of the invention, the resistor (s) may be connected to the busbars so that they can be closed (connected) and opened (disconnected). Thanks to this, it is possible to reduce the effort that the block faces, in relation to the continuous operation of the block. In one aspect of the invention, each of the resistor blocks is connected (closed) or disconnected (opens) according to a frequency that depends on the voltage value of the DC link. By means of the present, it is possible to optimize the reduction of energy with respect to the resistor block and the switches of the power converter. In one aspect of the invention, the resistor blocks are activated in succession as the value of the DC link voltage moves upward. By means of the present, it is possible to adapt the blocks to the relevant values of the fault situation. In one aspect of the invention, each of the resistor blocks is active for limited periods. Through this, it is possible to ensure that the blocks are not in operation for very long periods that result in damage to the control system. In one aspect of the invention, each of the resistor blocks is operated and activated with respect to its temperature. By means of the present, it is possible to control the blocks more precisely and thus prolong the active periods of the blocks. The invention also relates to a control system in which the system also contains at least two control units of the power converter, controlled with respect to at least one limiting value of the power converter during the failure. Through this, a control system that has advantages is established. In one aspect of the invention, the at least two units contain a plurality of resistor blocks and each block contains at least one resistor and one switch. Through this, it is possible to control the blocks individually and optimize the energy reduction. In one aspect of the invention, the resistor blocks further include a temperature measurement element. In one aspect of the invention, the at least two units also have the generator and the circuits, which are on the network side, of the power converter connected to each other by the DC link of the power converter. In one aspect of the invention, the system includes means for measuring the value of the voltage of the DC link and means for, in the power converter, comparing the value with the limit values, such as the value of the minimum or maximum limit voltage of the CC link. The invention also relates to wind turbines containing at least two units of the power converter controlled with respect to at least one limit value of the power converter. In one aspect of the invention, the at least two units of the power converter are located with respect to each other at a certain distance, for example, in different positions of the spindle. By means of the present, it is possible to compensate the influence of the heat of the different units, as well as to minimize the size of any necessary cooling means in each unit.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will be described below with reference to the figures, in which: Figure 1 illustrates a large wind turbine and 52-442 modern. Figure 2 illustrates an embodiment in accordance with the invention of a wind turbine generator having a frequency converter connected to an electrical distribution network. Figure 3 illustrates a section of the frequency converter. Figures 4a and 4b schematically illustrate the overvoltage control units and examples of the control signals of the units. Figure 5 illustrates a control system for overvoltage control units. Figure 6 illustrates a curve of the electrical distribution network voltage and the corresponding curve of the intermediate DC voltage when there is a fault in the distribution network. Figure 7 illustrates the control signals of the gate driver of the overvoltage units and the corresponding intermediate DC voltage curve. Figure 8 illustrates a temperature curve of the wind turbine during a fault in the distribution network.

DETAILED DESCRIPTION OF THE INVENTION Figure 1 illustrates a modern wind turbine 1, 52-442 which has a tower 2 and a spindle 3 located at the top of the tower. The rotor 5 of the wind turbine, which contains three wind turbine blades, is connected to the spindle by the low speed arrow extending outwardly from the front of the spindle. As illustrated in the figure, a wind current with a magnitude beyond a certain level will activate the rotor of the wind turbine due to the lift force induced on the blades, which causes them to rotate in a direction perpendicular to the wind. The rotation movement is converted into electrical energy, which is fed into the distribution network. Figure 2 illustrates a preferred embodiment of a variable speed wind turbine containing a dual-power induction electric generator y and a frequency and energy converter 12 connected to the generator rotor. The electric generator 6 contains a stator 7 connected to the distribution network by means of the disconnection switches 11 and the three-phase transformer 8, which can supply power, PSt (stator active energy) and QSt (stator reactive power), directly to the distribution network or receive it from the same network. The rotor (5 as illustrated in Figure 1) of the 52-442 wind turbine mechanically drives the rotor of the generator by means of the low speed arrow, the gear train and the high speed arrow (not illustrated in the figures). Additionally, the rotor is electrically connected to the frequency converter 12. The frequency converter 12 can convert a variable AC voltage into an intermediate DC voltage and, subsequently, into a fixed fixed frequency AC voltage. The frequency converter 12 includes a converter circuit 13 on the rotor side, which rectifies the AC voltage of the generator 6 and converts it into a DC voltage on the DC link 14 or inverts the DC voltage and converts it into a DC voltage. an AC voltage that will be supplied to the generator rotor. The DC link flattens or levels the DC voltage on a capacitor C of the DC link. The converter circuit 15 on the network side inverts the DC voltage and converts it into an AC voltage with a preferred frequency or vice versa. The rotor energy, Pr (active rotor energy) and Qr (reactive rotor energy) and the resulting AC voltage and preferred frequency are transferred to (or from) the distribution network through the transformer 8. The wind turbine it can be controlled so that it supplies the electric power of the generator to the network 52-442 with a constant voltage and frequency, without being affected by a changing wind or by the rotor speeds of the wind turbine. The DC link also contains at least two Bi and Bn overvoltage control units and resistor blocks connected between the two DC link distribution busbars. Each control unit is connected in parallel with the capacitor C of the DC link and contains at least one resistor R and a controllable power switch SP connected in series. The present embodiment of the control unit also contains an antiparallel diode for the resistor and the power switch. The power switch can be turned on and off to direct a current through the resistor and thus dissipate the power Pi, Pn of the resistor. The UDC voltage of the DC link can be reduced as the DC link capacitor removes the loads by directing the current through the resistor of the control unit. Consequently, the energy generated by the electric generator can be dissipated as power Pi, Pn in the overvoltage control units at intervals of time in which it is not possible to direct part or all of the PR energy towards the distribution network. Stator and rotor disconnect switches 11 allow the generator to be disconnected 52-442 of the distribution network, for example, when carrying out maintenance work on the wind turbine or on the distribution network, an insulation situation occurs. In addition, the wind turbine can be disconnected from the distribution network in the event that a network failure, involving a significant voltage drop, will persist for a long period. Figure 3 illustrates a section of the frequency converter that includes a branch of the converter circuit on the rotor side and the DC link. The branch is one of the three phases of the frequency converter with pulse width modulation or PWM, by "pulse width modulation", and includes two SP power switches, such as an insulated gate bipolar transistor or IGBT, per "insulated bipolar transistor gate ", with antiparallel diodes. The capacitor C of the DC link and the at least two Bi and Bn overvoltage control units are connected to the positive and negative distribution bars of the DC link. Additionally, the figure illustrates in schematic form the manner in which the power can be dissipated in the resistors of the at least two overvoltage control units Bi and Bn and, thus, reduce the voltage of the DC link. The switches of the 52-442 units are controlled in such a way that the power can be dissipated in the resistors either simultaneously or in different periods with respect to the value of the overvoltage and / or the temperature of the frequency converter included in the units, as will be explained additionally later. Figures 4a and 4b illustrate schematically the overvoltage control units with an example of the control signals Gl and G2 of the gate driver that control the units. Figure 4a illustrates one embodiment of the invention including four Bx-B overvoltage control units connected to the bus link system of the DC link 14 and in parallel with the capacitor C of the DC link. In schematic form it is illustrated that each of the control units contains a resistor R and a switch SP controlled by means of the control signal Gl or G2 of the gate exciter. The first control signal Gl is used to control the first two control units Bi and B2, that is, in different units arranged, for example, inside the spindle or the wind turbine tower the same amount of power is dissipated. power. The second control signal G2 is used to control the last two control units B3 and B, it is 52-442 say, in different locations the same amount of power dissipates. Figure 4b illustrates an example of the control signals Gl and G2 of the gate driver that control the units. The figure illustrates a first signal Gl that changes from a low value, off, to a high value, on, in a period thanks to which, the units Bi and B2 of overvoltage control will dissipate the power.

Subsequently, the second signal G2 changes from a low value, off, to a high value, on, in a period, thanks to which, the B3 and B4 units of overvoltage control will dissipate the power. The example shows that the overvoltage control units are controlled in such a way that they dissipate power in different periods, where the periods have a different duration, that is, they dissipate different amounts of power in the control units. However, for the individual control units several control strategies can be chosen, for example, by using resistors of equal or different value and applying the control for an equal or different period. By choosing the values of the resistor and the times, it is possible to divide the amount of energy that a control unit faces, that is, the same amount of energy for each unit or an amount 52-442 different. Figure 5 illustrates one embodiment of a control system of the overvoltage control units according to the invention. The system includes a variety of input values for the microprocessor μP of the measuring element, such as the value of the measured voltage UNet of the electrical distribution network, the value of the voltage UDc of the DC link of the frequency converter 12 and the temperature of the control units B? ~ Bn. The microprocessor also contains a connection to the PS storage of parameters and data, where the storage can contain the limit and threshold values, such as the maximum and minimum voltage values of the DC link and the temperature values. The maximum value defines the dangerous and potentially damaging overvoltage for the frequency converter switches. The minimum value defines the undervoltage that results in a dangerous and potentially damaging current that will flow through the switches of the frequency converter. The temperature limit values define the temperature values at which the control units or the frequency converter, as such, can be damaged. The limit values may also include values 52-442 of time, such as the longest time a control unit can be active and face the energy. In addition, the temperature or voltage threshold values can be stored in storage, where the values define a situation that must initiate some action such as activating more control units. Other values may be stored in the storage, such as overcurrent values, which indicate a shorter termination of the control signals of the frequency converter switches, for example, to limit the rotor current of a double induction generator. power of a wind turbine. The microprocessor controls several control units by the gate drivers GD? -GDn with respect to the measured and stored values. The figure illustrates that each gate driver controls two control units and, normally, with the same control signal of the gate driver is controlled to the switches of the control units. However, it should be understood that the microprocessor and a gate driver can individually control each control unit or that a single gate driver can control more than two units. A preferred mode of the control system 52-442 may include two or four control units, however, other numbers may be chosen if they imply any advantage for the given application, for example, more units in very high-energy frequency converters. Figure 6 illustrates an example of a voltage curve UNet of the electrical distribution network and the corresponding curve of the intermediate voltage UDC of the DC link when there is a fault in the distribution network. In the example, the voltage of the distribution network is illustrated schematically as a curve that falls rapidly from a nominal voltage value to a value very close to zero during the time of the network failure. The corresponding curve of the DC link voltage contains a slope due to the energy storage of the DC link capacitor. However, the value also drops and, finally, it reaches a UDCmin value, in which the rotor switches and the network-side converter circuits are deactivated and, thus, separates the frequency converter from the electrical generator and the distribution network. In addition, the control units connected between the DC link distribution busbars are deactivated and, consequently, the capacitor discharge of the DC link is stopped. In this way, the UDC voltage is maintained in 52-442 the UDCmin value until the fault of the distribution network has been corrected, the network voltage has returned to its nominal value and the UDC voltage has also returned to its normal value. The values of the initial current are restricted in this way while the voltage UDC is maintained at the UDCmin value, until the return of the network voltage. Figure 7 further illustrates the control signals Gi and G2 of the gate driver for several overvoltage control units of the control system and the corresponding curve of the intermediate DC voltage UDC during the failure of the network. The figure initially illustrates the way in which the network failure leads to an increase in the overvoltage to a value Ui (a value close to Umx). In order to protect the frequency converter and the wind turbine, the two control signals of the gate exciter reach a high value and, in this way, activate the corresponding control units. After a time interval, the voltage has cooled to a lower value U4 and a control signal reaches a low value; deactivating the corresponding control unit and, subsequently, the other control signal reaches a low value; deactivating the last control unit as the voltage continues to fall. When deactivating all 52-442 control units, the voltage increases again and the control system can once again activate one or more of the control units to control the voltage until the network fault has been corrected. Figure 8 shows the temperature curve of the control units of the wind turbine during a fault in the distribution network, in which the fault starts at time ti. The control unit or units are activated at that time and face an amount of energy due to their limitation of an overvoltage in the DC link of the frequency converter. Consequently, the temperature curve increases and, at time t2, the temperature limit value Tmax of the active control units is reached. The microprocessor activates additional control units and the temperature drops to a temperature limit value Tm? N at a time t3 and, consequently, at least one unit will be deactivated. This control over the number of active control units will continue until the network failure has been fixed. The invention has been exemplified in the foregoing with reference to specific examples. However, it should be understood that the invention is not limited to the particular examples described above, however, they may be used in connection with a wide variety of applications, for example, several 52-442 wind turbines connected to the same frequency converter. Additional applications may involve a synchronous or induction generator of a wind turbine connected to a full-scale frequency converter. Additionally, it should be understood that, in particular, the frequency converter can be designed in a multitude of varieties, for example, as a rectifier system, based on a thyristor, and inverter. It should be further understood that the invention can utilize a wide variety of measured values if they correspond directly or indirectly to the above-mentioned voltage and temperature values, for example, current values instead of voltage values. The position of the measurements of the wind turbine system can also be changed if the measurements correspond to those suggested above in the development, at least during the time of the network failure. 52-442 List 1. Wind turbine. 2. Tower of the wind turbine. 3. Spindle of the wind turbine. 4. Cube of the wind turbine. 5. Rotor of the wind turbine. 6. Induction generator. 7. Stator side of the generator that includes the connections with the disconnection switches and the network transformer. 8. Transformer of the distribution network. 9. Distribution network that has the voltage UNet- 10. Generator rotor side that includes the connections with the frequency converter. 12. Frequency converter. 13. Converter side rotor circuit. 14. DC link between the rotor and the converter circuits on the side of the network. 15. Converter circuit on the network side. 16. Connection of the converter with the disconnection switches and the network transformer. 17. Control system for overvoltage control units. Bn Overvoltage control unit no. n. C. CC link capacitor. 52-442 D. Anti-parallel diode with a power switch. In. Activate the control signal. Gn. Control signal no. n of the gate. GDn Gate exciter unit no. n. I. Current. PR, QR Flow of active and reactive energy of the rotor. 3st, Qst Stator active and reactive energy flow. Pi, Pn Energy flow through the control units during an overvoltage situation. . Storage of parameters / data. R. Resistor. SP Power switch, such as a bipolar gate transistor isolated or IGBT, by "Insulated Gate Bipolar Transistor", t. Time [seconds]. T. Temperature [degrees Celsius]. U. Voltage [volts]. UNet Voltage of the distribution network. UDC Voltage in the DC link. 52-442

Claims

CLAIMS: 1. A method to control a wind turbine, the wind turbine contains an electric generator and a power converter, connected to an electrical distribution network during a network failure, the method comprises the steps of: detecting the failure of the electrical distribution network and operating at least two control units of the power converter with respect to at least one limit value of the power converter and to at least one other additional value, such as the temperature of the control units or of the power converter. The method for controlling a wind turbine according to claim 1, wherein the at least two control units are operated with respect to a minimum or maximum voltage limit value of the DC link of the power converter to maintain the value of the DC link voltage between the minimum and maximum voltage limit values. 3. The method for controlling a wind turbine according to claim 1, wherein the control units include the generator and the circuits, which are on the network side, of the power converter, which come into operation to disconnect from the electrical generator and from the electrical distribution network to the power converter at the moment when the minimum or maximum limit value of the DC link is reached. 4. The method for controlling a wind turbine according to any of claims 1 to 3, wherein the control units further contain one or more resistor blocks, which connect at least one resistor between the distribution bars of the DC link of the power converter. The method for controlling a wind turbine according to claim 4, wherein the resistor (s) can be connected to the bus bars in such a way that they can be connected (closed) or disconnected (opened). 6. The method for controlling a wind turbine according to claim 4 or 5, wherein each of the resistor blocks can be switched on or off according to a frequency that depends on the voltage value of the DC link. The method for controlling a wind turbine according to any of claims 4 to 6, wherein the resistor blocks are activated successively as the voltage value of the DC link moves upward. 8. The method for controlling a wind turbine according to any of claims 4 to 7, where each of the resistor blocks is active for limited time intervals. 9. The method for controlling a wind turbine according to any of claims 4 to 8, where each of the resistor blocks is put to work and is active in relation to the temperature of the blocks. 10. A control system for controlling a wind turbine connected to an electrical distribution network during a network failure, the system contains: a means to detect the failure of the electrical distribution network and a power converter characterized in that the system further contains at least two control units (B? -Bn) of the controlled power converter, during the fault, with respect to at least one limit value of the power converter and at least one additional value, such as the temperature of the power converter. the control units or the power converter. The control system according to claim 10, characterized in that the at least two units contain a plurality of resistor blocks, wherein each block includes at least one resistor (R) and one switch (SP). The control system according to claim 11, characterized in that the resistor blocks also include temperature measuring elements. The control system according to any of claims 10 to 12, characterized in that the at least two units also have the generator and the circuits, which are on the network side, of the power converter connected to each other by the DC link of the power converter. The control system according to any of claims 10 to 13, characterized in that the system includes means for measuring the value of the voltage of the DC link and means for, in the power converter, comparing the value with the limit values , such as the value of the minimum or maximum limit voltage of the DC link. 15. A wind turbine connected to a distribution network (9), the wind turbine contains: an electric generator (6), a power converter (12) connected to the electric generator and the distribution network, and a control system for according to any of claims 10 to 14, the system 52-442 contains at least two units of the controlled power converter with respect to at least one limit value of the power converter and to at least one other additional value, such as the temperature of the control units or power converter. 16. The wind turbine according to claim 15, wherein the at least two units (Bi-Bn) of the power converter are located with respect to each other at a distance, for example, at different positions of the spindle. 52-442